The primary objectives are: a) the chemical and biochemical characterization of alpha-amanitin conjugates to bovine and human immunoglobulin G. Conjugation will be carried out by carbodiimide coupling of derivatives of alpha-amanitin with either reactive amino or carboxyl groups to carboxyl and amino groups on the proteins. The preparation and characterization of the N-hydroxysuccinimide ester of alpha-amanitin-diazobenzoylglycylglycine will be carried out, followed by the characterization of conjugates to amino groups on immunoglobulin G. All conjugates will be evaluated with respect to inhibition of calf thymus RNA polymerase II and non specific cell toxicity. b) The chemical and biochemical characterization of conjugates of alpha-amanitin to lectins with different ligand specificity (Concanavalin A, wheat germ agglutinin, soybean lectin). c) The evaluation of these conjugates with respect to cell toxicity toward specific cell types as a function of the density of cell surface receptors for a given lectin acting as the carrier for the alpha-amanitin. d) The preparation of alpha-amanitin-immunoglobulin G conjugates in which the immunoglobulin is specifically directed to theta antigen on EL4 (mouse lymphocytic leukemia) cells and mouse T lymphocytes. e) Examination of the potential of the amanitin anti theta immunoglobulins to specifically inhibit cell proliferation (DNA synthesis) of EL4 cells in vitro, compared with C1498 cells (mouse myeloid leukemia, lacking theta antigen) and f) Examination of the potential of the amanitin-anti theta immunoglobulin to arrest the proliferation of EL4 cells in innoculated mice.